Examples of ring laser gyroscopes are shown and described in U.S. Pat. No. 3,373,650 issued to J. Killpatrick and U.S. Pat. No. 3,390,606 issued to T. Podgorski. An integral part of a ring laser gyro is the laser beam source or generator. One type of laser generator comprises electrodes and a gas discharge cavity in combination with a plurality of mirrors which establishes an optical closed loop path. The gas discharge cavity is generally formed by a laser block having a plurality of interconnecting tunnels or bores.
Present day ring laser gyros employ a gas discharge cavity filled with a lasing gas which is ionized when excited by an electric current passing from one electrode to another through the lasing gas. If the plurality of mirrors are properly aligned, two counter-propagating laser beams will be established, traveling in opposite directions along the optical closed loop path. Each counter-propagating laser beam may consist of several light beams sometimes referred to as spatial modes. The centermost mode, commonly referred to as the TEM.sub.00 mode (and also referred to as the fundamental or primary spatial mode), contains the greatest amount of energy and is of greatest value to the operation of the ring laser gyro.
One embodiment of a ring laser gyro system includes a device called a path length controller that is capable of making slight alterations to the length of the optical closed loop path by changing the distance between the plurality of mirrors. To ensure that the path length is properly set, a laser intensity monitor is appropriately coupled to the discharge cavity in order to observe the intensity of a portion of one of the counter-propagating laser beams exiting through one of the plurality of mirrors. Desirably, the laser intensity monitor should be sensitive to only the TEM.sub.00 mode of the laser beam exiting the mirror. Based on the intensity of the TEM.sub.00 mode, the path length is regulated so that the TEM.sub.00 mode always contains the maximum amount of energy possible.
To achieve this, only the TEM.sub.00 mode of one of the counter-propagating laser beams is monitored. If more than one spatial mode was monitored simultaneously, the ring laser gyro might attempt to adjust the path length so as to maximize the energy in a mode other than the TEM.sub.00 mode. This would cause the ring laser gyro to give less precise readings, than if only the TEM.sub.00 mode was being monitored.
Heretofore, a laser intensity monitoring apparatus consisted of a photodetector contained within a package which comprised an enclosure in which the photodetector is mounted. The enclosure further included a transparent window generally parallel to, and in front of, the photosensitive surface of the photodetector. A mylar mask is attached to the outer surface of the transparent window with an adhesive. The mylar mask is similar to a photographic negative which is generally opaque with an aperture of a size and shape that will only allow the TEM.sub.00 mode to pass through.
Generally, the photodetector package is rigidly attached to the mirror substrate having a partially transmissive mirror. The mirror substrate is rigidly fixed to the laser block which provides the discharge cavity. The photodetector package is appropriately positioned relative to the portion of the laser beam exiting through the mirror, as aforesaid, such that the photodetector is responsive to the laser beam.
Unfortunately, this assembly with the mylar mask has presented several problems. First, great difficulty is encountered in getting the mask to remain flat on the transparent window. Because an adhesive is used to attach the mask to the window, air bubbles, which may cause deleterious optical effects, become trapped between the mask and the window. Moreover, the ends of the mask also have a tendency to curl. These effects are further exacerbated when the monitor is subjected to changing temperature because of the difference between the mylar mask's and the window's thermal coefficients of expansion. Further, not only is the mylar mask difficult to clean, but it also fogs when exposed to radiation. In addition, imperfections of the mylar mask can cause light scattering leading to gyro performance errors.